Airfoil having environmental barrier topcoats that vary in composition by location

ABSTRACT

An airfoil includes an airfoil wall that defines a leading end, a trailing end, and suction and pressure sides that join the leading end and the trailing end. The airfoil wall is formed of a silicon-containing ceramic. A first environmental barrier topcoat is disposed on the suction side of the airfoil wall, and a second, different environmental barrier topcoat is disposed on the pressure side of the airfoil wall. The first topcoat is vaporization-resistant and the second topcoat is resistant to calcium-magnesium-aluminosilicate.

BACKGROUND

Components in a gas turbine engine often include barrier coatings toprotect the underlying component from the effects of the severeoperating environment. Barrier coatings are available in numerousvarieties, which can include thermal barrier coatings and environmentalbarrier coatings. Thermal barrier coatings are typically designed formaximizing thermal insulation of a component from the surroundinghigh-temperature environment. Environmental barrier coatings aretypically designed for maximizing resistance of infiltration or attackby the environment.

SUMMARY

An airfoil according to an example of the present disclosure includes anairfoil wall that defines a leading end, a trailing end, and suction andpressure sides that join the leading end and the trailing end. Theairfoil wall is formed of a silicon-containing ceramic. A firstenvironmental barrier topcoat is disposed on the suction side of theairfoil wall. A second, different environmental barrier topcoat isdisposed on the pressure side of the airfoil wall.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is of a composition that is selected fromHfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, alkaline earthalumino-silicates, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the secondenvironmental barrier topcoat is of a composition that is selected froma mixture of HfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂,Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ and calcium aluminosilicate,Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is of a composition that is selected fromHfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, alkaline earthalumino-silicates, and combinations thereof, and the secondenvironmental barrier topcoat is of a composition that is selected froma mixture of HfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂,Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ and calcium aluminosilicate,Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is multi-layered and includes anover-layer of a composition that is selected from HfO₂, Y₂SiO₅, YbSiO₅,and combinations thereof and an under-layer of a composition that isselected from HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the secondenvironmental barrier topcoat is multi-layered and includes anover-layer selected from a mixture of HfSiO₄ and calciumaluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ andcalcium aluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinationsthereof and an under-layer of a composition of a mixture of HfSiO₄ andcalcium aluminosilicate.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is multi-layered and includes anover-layer of a composition that is selected from HfO₂, Y₂SiO₅, YbSiO₅,and combinations thereof and an under-layer of a composition that isselected from HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, and combinations thereof, andthe second environmental barrier topcoat is multi-layered and includesan over-layer of a composition of Gd₂Hf₂O₇ and an under-layer of acomposition of a mixture of HfSiO4 and calcium aluminosilicate.

A further embodiment of any of the foregoing embodiments includes athird environmental barrier topcoat disposed on the leading end of theairfoil wall.

In a further embodiment of any of the foregoing embodiments, the thirdenvironmental barrier topcoat is of a composition selected from yttriastabilized zirconia, ZrO²⁻YO_(1.5)TaO_(2.5) and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is of a composition that is selected fromHfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, alkaline earthalumino-silicates (AEAl₂Si₂O₈), and combinations thereof, and the secondenvironmental barrier topcoat is of a composition that is selected froma mixture of HfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂,Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ and calcium aluminosilicate,Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the firstenvironmental barrier topcoat is multi-layered and includes anover-layer of a composition that is selected from HfO₂, Y₂SiO₅, YbSiO₅,and combinations thereof and an under-layer of a composition that isselected from HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, and combinations thereof, andthe second environmental barrier topcoat is multi-layered and includesan over-layer of a composition of Gd₂Hf₂O₇ and an under-layer of acomposition of a mixture of HfSiO₄ and calcium aluminosilicate.

A further embodiment of any of the foregoing embodiments includes afourth environmental barrier topcoat disposed in the trailing end of theairfoil wall, wherein the fourth environmental barrier topcoat isthicker than each of the first environmental barrier topcoat and thesecond environmental barrier topcoat.

A further embodiment of any of the foregoing embodiments includes afourth environmental barrier topcoat disposed in the trailing end of theairfoil wall, wherein the fourth environmental barrier topcoat has, byvolume percent, a higher porosity than each of the first environmentalbarrier topcoat and the second environmental barrier topcoat.

An airfoil according to an example of the present disclosure includes anairfoil wall that defines a leading end including an apex, a trailingend, and suction and pressure sides that join the leading end and thetrailing end. The airfoil wall is formed of a silicon-containingceramic. A vaporization-resistant environmental barrier topcoat of afirst composition is disposed on the suction side of the airfoil wall.The vaporization-resistant environmental barrier topcoat initiates atthe leading end at a first distance from the apex. Acalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat of a second, different composition is disposed on the pressureside of the airfoil wall. Thecalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat initiates at the leading end at a second distance from the apex

In a further embodiment of any of the foregoing embodiments, the firstdistance is from 0.5 millimeters to 7.7 millimeters and the seconddistance is from 0.5 millimeters to 7.7 millimeters.

In a further embodiment of any of the foregoing embodiments, the firstdistance is from 1.2 millimeters to 3 millimeters and the seconddistance is from 1.2 millimeters to 3 millimeters.

In a further embodiment of any of the foregoing embodiments, thevaporization-resistant environmental barrier topcoat is of a compositionthat is selected from HfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇,Yb₂Si₂O₇, alkaline earth alumino-silicates (AEAl₂Si₂O₈), andcombinations thereof, and thecalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat is of a composition that is selected from a mixture of HfSiO₄and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixture ofHfO₂, HfSiO₄ and calcium aluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇,and combinations thereof.

In a further embodiment of any of the foregoing embodiments, thevaporization-resistant environmental barrier topcoat is multi-layeredand includes an over-layer of a composition that is selected from HfO₂,Y₂SiO₅, YbSiO₅, and combinations thereof and an under-layer of acomposition that is selected from HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, andcombinations thereof, and thecalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat is multi-layered and includes an over-layer of a composition ofGd₂Hf₂O₇ and an under-layer of a composition of HfSiO₄ and calciumaluminosilicate.

A further embodiment of any of the foregoing embodiments includes anadditional environmental barrier topcoat disposed on the trailing end ofthe airfoil wall, the additional environmental barrier coatinginitiating at a distance from a mechanical trailing edge of the airfoilthat is greater than the thickness of the mechanical trailing edge by afactor of 2 to 10.

A gas turbine engine according to an example of the present disclosureincludes a compressor section, a combustor in fluid communication withthe compressor section, and a turbine section in fluid communicationwith the combustor. The turbine section has an airfoil according to theforegoing embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example gas turbine engine.

FIG. 2 illustrates an example airfoil of the gas turbine engine.

FIG. 3 illustrates a sectioned view of an airfoil that has first andsecond environmental barrier topcoats.

FIG. 4 illustrates a sectioned view of the first environmental barriertopcoat.

FIG. 5 illustrates a sectioned view of the second environmental barriertopcoat.

FIG. 6 illustrates a sectioned view of a multi-layered firstenvironmental barrier topcoat.

FIG. 7 illustrates a sectioned view of a multi-layered secondenvironmental barrier topcoat.

FIG. 8 illustrates a sectioned view of another example airfoil thatadditionally has a third environmental barrier topcoat.

FIG. 9 illustrates a sectioned view of the third environmental barriertopcoat.

FIG. 10 illustrates a sectioned view of another example airfoil thatadditionally includes a fourth environmental barrier topcoat.

FIG. 11 illustrates a sectioned view through a portion of the fourthenvironmental barrier topcoat.

FIG. 12 illustrates a sectioned view through another portion of thefourth environmental barrier topcoat.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 drivesair along a bypass flow path B in a bypass duct defined within a nacelle15, and also drives air along a core flow path C for compression andcommunication into the combustor section 26 then expansion through theturbine section 28. Although depicted as a two-spool turbofan gasturbine engine in the disclosed non-limiting embodiment, it should beunderstood that the concepts described herein are not limited to usewith two-spool turbofans as the teachings may be applied to other typesof turbine engines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a first (or low) pressure compressor 44 and a first (orlow) pressure turbine 46. The inner shaft 40 is connected to the fan 42through a speed change mechanism, which in exemplary gas turbine engine20 is illustrated as a geared architecture 48 to drive a fan 42 at alower speed than the low speed spool 30. The high speed spool 32includes an outer shaft 50 that interconnects a second (or high)pressure compressor 52 and a second (or high) pressure turbine 54. Acombustor 56 is arranged in exemplary gas turbine 20 between the highpressure compressor 52 and the high pressure turbine 54. A mid-turbineframe 57 of the engine static structure 36 may be arranged generallybetween the high pressure turbine 54 and the low pressure turbine 46.The mid-turbine frame 57 further supports bearing systems 38 in theturbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of the low pressure compressor, or aftof the combustor section 26 or even aft of turbine section 28, and fan42 may be positioned forward or aft of the location of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1 and less than about 5:1. Itshould be understood, however, that the above parameters are onlyexemplary of one embodiment of a geared architecture engine and that thepresent invention is applicable to other gas turbine engines includingdirect drive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and35,000 ft (10,668 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (‘TSFC’)”—is the industry standard parameter of lbm of fuelbeing burned divided by 1 bf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram ° R)/(518.7° R)]{circumflex over( )}0.5. The “Low corrected fan tip speed” as disclosed herein accordingto one non-limiting embodiment is less than about 1150 ft/second (350.5meters/second).

FIG. 2 illustrates a representative example of an airfoil 60 used in theturbine engine 20 (see also FIG. 1), and FIG. 3 illustrates a sectionedview of the airfoil 60. As shown, the airfoil 60 is a turbine blade;however, it is to be understood that, although the examples herein maybe described and shown with reference to turbine blades, this disclosureis also applicable to turbine vanes as well as cooled blades or vanes inlocations other than shown.

The airfoil 60 includes an (outer) airfoil wall 62 that delimits theaerodynamic profile of the airfoil 60. In this regard, the wall 62defines a leading end 62 a (see FIG. 3), a trailing end 62 b, and firstand second sides 62 c/62 d that join the leading end 62 a and thetrailing end 62 b. In this example, the first side 62 c is a suctionside and the second side 62 d is a pressure side. The airfoil wall 62generally extends in a longitudinal direction, which in the engine 20 isan axial direction relative to the central engine axis A. For a blade,the airfoil wall 62 will typically span from an inner platform to a freetip end. In a vane, the airfoil wall 62 will typically span from aninner platform to an outer platform.

The airfoil wall 62 is formed of a ceramic material and, in particular,a silicon-containing ceramic. An example silicon-containing ceramic issilicon carbide (SiC). In a further example, the airfoil wall 62 isformed of a SiC/SiC ceramic matrix composite in which SiC fibers aredisposed within a SiC matrix.

An airfoil may be exposed to relatively severe environmental conditionsduring operation. Such conditions can reduce the durability ofsilicon-containing ceramics, such as silicon carbide. In this regard,silicon-containing ceramic airfoils can include an environmental barriercoating (“EBC”) that is designed to protect the underlying ceramic fromthe conditions and, in particular, resist infiltration or attack byenvironmental substances. The local conditions across an airfoil can,however, significantly vary and thus challenge an EBC that is designedfor best performance in average or typical conditions. In particular,conditions can vary between the suction and pressure sides with regardto volatilization conditions and calcium-magnesium-aluminosilicate(“CMAS”) exposure. Volatilization occurs when silicon in an EBC reactsand is converted to a gaseous product that results in material loss andreduction in structural integrity. Dirt/debris that deposits on anairfoil surface can be molten at times, and this viscous liquid canreact with and wick into an EBC and ultimately cause spallation. Theairfoil leading edge and outboard ˜75-100% span, forward suction sidemay also be exposed to more severe conditions of foreign and bill ofmaterial object impact, and erosion than the remainder of the airfoil,and temperature conditions may be highest at the trailing end. In theseregards, as will be described in more detail below, the airfoil 60includes EBCs that selectively vary in composition by location over theairfoil 60 in order to enhance local environmental protection of theairfoil 60.

The example in FIG. 3 relates to the varying conditions between thesuction side 62 c and the pressure side 62 d with regard tovolatilization conditions and CMAS exposure. For instance,volatilization conditions are more severe at the suction side 62 c dueto the higher local gas velocities at the suction side 62 c incomparison to the pressure side 62 d. And CMAS exposure is more severeat the pressure side 62 d due to contact with dirt/debris that impactthe pressure side 62 d. In these regards, the airfoil 60 includes afirst or vaporization-resistant environmental barrier topcoat 64(hereafter “first topcoat 64”) that is disposed on the suction side 62 cand a second or CMAS-resistant environmental barrier topcoat 66(hereafter “second topcoat” 66”) disposed on the pressure side 62 d thatis of different composition than the topcoat 64. It is to be appreciatedthat as used herein the term “topcoat” refers to an external coatingthat is directly exposed to the surrounding environment of the airfoil60. Such a topcoat may be a single layer or multilayer structure, andmay be directly disposed on the underlying airfoil wall 62 (with noover-layers) or overlie one or more coating under-layers.

In examples, the first topcoat 64 is of a composition that is selectedfrom HfO₂, rare earth monosilicate (RESiO₅), HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇,alkaline earth alumino-silicates (AEAl₂Si₂O₈) and combinations thereof.Rare earth elements include cerium (Ce), dysprosium (Dy), erbium (Er),europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium(Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm),scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium(Y). In additional examples, the rare earth silicate is Y₂SiO₅, YbSiO₅,or a combination thereof. Alkaline earth elements include magnesium(Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Anexample of an alkaline earth alumino silicate isBa_(0.75)Sr_(0.25)Al₂Si₂O₈ (BSAS). In additional examples, the firsttopcoat 64 is of a composition that is selected from HfO₂, Y₂SiO₅,YbSiO₅, or combinations thereof. The HfO₂, excludes silicon, and siliconvolatilization is thus avoided. The Y₂SiO₅ and YbSiO₅ include siliconbut the silicon in these compounds has a low vapor pressure. The HfSiO₄,Y₂Si₂O₇, and Yb₂Si₂O₇ may also be used. The silicon in Y₂SiO₅ and YbSiO₅has a higher silicon vapor pressure but the activity of the silicon isrelatively low in comparison to pure SiO₂.

The second topcoat 66 is of a composition that is selected from amixture of HfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇,a mixture of HfO₂, HfSiO₄ and calcium aluminosilicate, Y₂Si₂O₇,Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof. The first topcoat 64 isgenerally formulated to either exclude silicon, in which case siliconvolatilization is avoided, or include silicon in a low vapor pressurecompound. Such a low vapor pressure compound reduces the propensity ofthe silicon to volatilize in the low pressure conditions at the suctionside 62 c. In general, the vapor pressure of the silicon (if present) inthe first topcoat 64 is at least 10% lower than the vapor pressure ofthe silicon (if present) in the second topcoat 66. The second topcoat66, on the other hand, is formulated for higher density than the firsttopcoat 64 and/or for chemical compatibility with CMAS in order to blockor hinder infiltration of the CMAS.

FIG. 4 illustrates a representative sectioned view of a further exampleof the first topcoat 64. In this example, a bondcoat 68 is disposedbetween the first topcoat 64 and the airfoil wall 62 (i.e., substrate).For example, the bondcoat 68 facilitates adherence of the first topcoat64 to the airfoil wall 62 and/or provides additional environmentalprotection. The bondcoat 68 may be a single layer or multi-layered. Asan example, the bondcoat 68 may include silicon (elemental), metalsilicides (e.g., MoSi₂ Nb₅Si₃ and YSi₂), and composite bond coats inwhich oxygen getter phases are disposed in a glass and/or ceramicmatrix.

Likewise, FIG. 5 illustrates a representative sectioned view of afurther example of the second topcoat 66. In this example, the bondcoat68 is also disposed between the second topcoat 66 and the airfoil wall62 (i.e., substrate). The bondcoat 68 may be a single layer ormulti-layered and may be of the compositions described above.

In further examples, the first topcoat 64, the second topcoat 66, orboth may be multi-layered. For example, FIG. 6 illustrates amulti-layered structure of the first topcoat 64, including an over-layer64 a and an under-layer 64 b. The over-layer 64 a is of a compositionthat is selected from HfO₂, Y₂SiO₅, YbSiO₅, and combinations thereof,and the under-layer 64 b is of a composition that is selected fromHfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, and combinations thereof. In general theover-layer 64 b provides enhanced resistance to volatilization. Theunder-layer 64 b may have a lower resistance to volatilization butbetter resistance to CMAS. In an additional example, the thermalexpansion coefficient of the HfSiO₄, Y₂Si₂O₇, and/or Yb₂Si₂O₇ ofunder-layer 64 b closely matches the SiC of the airfoil wall 62 than thematerials of the over-layer 64 a. Thus, in circumstances where it isdesired to increase the total thickness of the first topcoat 64, themechanical stability can be increased by increasing the thickness of theunder-layer 64 b with respect to the over-layer 64 a. In one variation,the first topcoat 64 is multi-layered and includes the second topcoat 66as an under-layer. That is, the under-layer 64 b may be any of thecompositions disclosed herein for the second topcoat 66 and theover-layer 64 a may be any of the compositions disclosed herein for thetopcoat 64. Such a configuration may be fabricated by apply the secondtopcoat 66 to the entire airfoil 60, and then applying the first topcoat64 on the suction side of the airfoil 60 such that the topcoat 66 on thesuction side becomes the under-layer 64 b and the topcoat 64 becomes theover-layer 64 a.

FIG. 7 illustrates a multi-layered structure of the second topcoat 66,including an over-layer 66 a and an under-layer 66 b. The over-layer 66a may be a composition as disclosed above for the second topcoat 66,such as a composition of Gd₂Hf₂O₇, and the under-layer 66 b is of acomposition of HfSiO₄ and calcium aluminosilicate or HfSiO₄ or Yb₂Si₂O₇or Y₂Si₂O₇. For instance, the calcium aluminosilicate is in discreteregions that are dispersed in the HfSiO₄ matrix. The over-layer 66 a andthe under-layer 66 b may serve to provide different levels of CMASresistance.

FIG. 8 illustrates a further example of an airfoil 160 that may be usedin the engine 20. In this disclosure, like reference numerals designatelike elements where appropriate and reference numerals with the additionof one-hundred or multiples thereof designate modified elements that areunderstood to incorporate the same features and benefits of thecorresponding elements. The airfoil 160 is similar to the airfoil 60 butadditionally includes a third environmental barrier topcoat 70 (“thirdtopcoat 70”) that is disposed on the leading edge and outboard forwardsuction side 62 c of the airfoil wall 62. For instance, the thirdtopcoat 70 has a composition that is configured to enhance resistance toimpact and erosion. For example, the third topcoat 70 is of acomposition selected from yttria stabilized zirconia, (1−2x)ZrO₂,xYO_(1.5), xTaO_(2.5), and combinations thereof. In one example, theyttria stabilized zirconia is stabilized with, by weight percent,approximately 7% of the yttria (i.e. 7YSZ). In one example, the(1−2x)ZrO₂, xYO_(1.5), xTaO_(2.5), contains, by atomic ratio, “x”ranging from 5 to 25%

FIG. 9 illustrates a representative sectioned view of a further exampleof the third topcoat 70. In this example, the bondcoat 68 is alsodisposed between the third topcoat 70 and the airfoil wall 62 (i.e.,substrate). The bondcoat 68 may be a single layer or multi-layered andmay be of the compositions described above.

FIG. 10 illustrates a further example of an airfoil 260 that may be usedin the engine 20. The airfoil 260 is similar to the airfoil 160 butadditionally includes a fourth environmental barrier topcoat 72 a/72 bthat is disposed on the trailing end 62 b of the airfoil wall 62. Forinstance, the fourth topcoat 72 a/72 b is configured to enhance thermalresistance at the trailing end 62 b. In this regard, the fourth topcoat72 a is thicker than each of the first topcoat 64 and the second topcoat66 in order to provide enhanced thermal barrier. Additionally oralternatively, the fourth topcoat 72 a/72 b may have, by volume percent,a higher porosity than each of the first topcoat 64 and the secondtopcoat 66.

As an example, FIG. 11 shows the fourth topcoat 72 a adjacent the firsttopcoat 64. In this example, the fourth topcoat 72 a is of the samecomposition as the first topcoat 64 and also includes the bondcoat 68.The fourth topcoat 72 a has a thickness t1 and the first topcoat 64 hasa thickness t2. The thickness t1 is from 110% to 500% of t2. Similarly,the bondcoat 68 underlying the fourth topcoat 72 a may be thicker thanthe bondcoat 68 underlying the first topcoat 64. The fourth topcoat 72 amay also be segmented or have other features for stress cracking relief.

The example in FIG. 12 shows the fourth topcoat 72 b adjacent the secondtopcoat 66. In this example, the fourth topcoat 72 b is of the samecomposition as the second topcoat 66 and also includes the bondcoat 68.The fourth topcoat 72 b has a thickness t3 and the second topcoat 66 hasa thickness t4. The thickness t3 is from 110% to 500% of t4. Similarly,the bondcoat 68 underlying the fourth topcoat 72 b may be thicker thanthe bondcoat 68 underlying the second topcoat 66. The fourth topcoat 72b may also be segmented or have other features for stress crackingrelief.

Alternatively or additionally, the fourth topcoat 72 a/72 b may have ahigher porosity, by volume percent, than each of the first topcoat 64and the second topcoat 66. The higher porosity serves to increase thethermal insulating effect of the fourth topcoat 72 a/72 b to provideenhanced thermal resistance at the trailing end 62 b. For example, theporosity of the fourth topcoat 72 a/72 b is higher than the porosity ofeach of the first topcoat 64 and the second topcoat 66 by a factor of1.1 to 5. As an example, the first topcoat 64 may have a porosity of0.5-20%, and especially from 1-5%. The fourth topcoat 72 a/2 b may havea porosity of 1-50%, and especially 5-15%.

In further examples, the particular locations of the topcoats64/66/70/72 a/72 b on the airfoil wall 62 are controlled to correspondto the local variations in the environmental conditions. For instance,the locations may be represented by transition points (or ranges) atwhich the conditions change. Such transition points or ranges may berelative to one or more reference locations or reference conditions.Referring to FIG. 10, the first topcoat 64 initiates at point 1 andterminates at point 3; the second topcoat initiates at point 2 andterminates at point 4; the third topcoat 70 initiates at point 1 andterminates at point 2; and the fourth topcoats 72 a and 72 b initiateat, respectively, points 3 and 4. For instance, point 1 represents thelocation where the leading end 62 a transitions to the suction side 62c, point 2 represents the location where the leading end 62 atransitions into the pressure side 62 d (and this region from point 1 topoint 2 is defined by curvature values in excess of 1.5), point 3represents the location where the suction side 62 c transitions into thetrailing end 62 b, and point 4 represents the location where thepressure side 62 d transitions into the trailing end 62 d. Points 1 and2 may be located relative to the apex of the leading end 62 a, which maybe the stagnation point of the airfoil. The stagnation point is thelocation at which the local flow velocity is zero. In examples, points 1and 2 are both located from 0.5 millimeters to 7 7 millimeters along thesurface of the airfoil from the apex, such as from 1.2 millimeters to3.5 millimeters. Points 3 and 4 may be located with reference to adistance from the mechanical trailing edge. For example, the distance isrelative to a thickness at the mechanical trailing edge (withoutcoatings). In one example, the distance is greater than the thickness bya factor of 2 to 10. At each of the points 1, 2, 3, and 4 the respectivetopcoats may discretely abut, overlap, or transition in a graded manner

As shown, point 1 is toward the suction side of the airfoil 260 andpoint 2 is toward the pressure side of the airfoil 260. In variations,point 1, point 2, or both may be inverted. For instance, an invertedpoint 1 is located from 0.5 millimeters to 7.7 millimeters along thesurface of the airfoil from the apex toward the pressure side, and aninverted point 2 is located from 0.5 millimeters to 7.7 millimetersalong the surface of the airfoil from the apex toward the suction side.As a result of the inversion, the associated coating in essence “wrapsaround” the leading end 62 a. If both points 1 and 2 are inverted, theassociated coatings would overlap over the leading end 62 a.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthis disclosure. The scope of legal protection given to this disclosurecan only be determined by studying the following claims.

What is claimed is:
 1. An airfoil comprising: an airfoil wall defining aleading end, a trailing end, and suction and pressure sides joining theleading end and the trailing end, the airfoil wall being formed of asilicon-containing ceramic; a first environmental barrier topcoatdisposed on the suction side of the airfoil wall; and a second,different environmental barrier topcoat disposed on the pressure side ofthe airfoil wall.
 2. The airfoil as recited in claim 1, wherein thefirst environmental barrier topcoat is of a composition that is selectedfrom HfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, alkalineearth alumino-silicates (AEAl₂Si₂O₈), and combinations thereof.
 3. Theairfoil as recited in claim 1, wherein the second environmental barriertopcoat is of a composition that is selected from a mixture of HfSiO₄and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixture ofHfO₂, HfSiO₄ and calcium aluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇,and combinations thereof.
 4. The airfoil as recited in claim 1, whereinthe first environmental barrier topcoat is of a composition that isselected from HfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇,alkaline earth alumino-silicates (AEAl₂Si₂O₈), and combinations thereof,and the second environmental barrier topcoat is of a composition that isselected from a mixture of HfSiO₄ and calcium aluminosilicate,Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ and calciumaluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof.5. The airfoil as recited in claim 1, wherein the first environmentalbarrier topcoat is multi-layered and includes an over-layer of acomposition that is selected from HfO₂, Y₂SiO₅, YbSiO₅, and combinationsthereof and an under-layer of a composition that is selected fromHfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, and combinations thereof.
 6. The airfoil asrecited in claim 1, wherein the second environmental barrier topcoat ismulti-layered and includes an over-layer selected from a mixture ofHfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixtureof HfO₂, HfSiO₄ and calcium aluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇,Gd₂Si₂O₇, and combinations thereof and an under-layer of a compositionof a mixture of HfSiO₄ and calcium aluminosilicate.
 7. The airfoil asrecited in claim 1, wherein the first environmental barrier topcoat ismulti-layered and includes an over-layer of a composition that isselected from HfO₂, Y₂SiO₅, YbSiO₅, and combinations thereof and anunder-layer of a composition that is selected from HfSiO₄, Y₂Si₂O₇,Yb₂Si₂O₇, and combinations thereof, and the second environmental barriertopcoat is multi-layered and includes an over-layer of a composition ofGd₂Hf₂O₇ and an under-layer of a composition of a mixture of HfSiO₄ andcalcium aluminosilicate.
 8. The airfoil as recited in claim 1, furthercomprising a third environmental barrier topcoat disposed on the leadingend of the airfoil wall.
 9. The airfoil as recited in claim 8, whereinthe third environmental barrier topcoat is of a composition selectedfrom yttria stabilized zirconia, ZrO₂YO_(1.5)TaO_(2.5), and combinationsthereof.
 10. The airfoil as recited in claim 9, wherein the firstenvironmental barrier topcoat is of a composition that is selected fromHfO₂, rare earth monosilicate, HfSiO₄, Y₂Si₂O₇, Yb₂Si₂O₇, alkaline earthalumino-silicates (AEAl₂Si₂O₈), and combinations thereof, and the secondenvironmental barrier topcoat is of a composition that is selected froma mixture of HfSiO₄ and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂,Gd₂Hf₂O₇, a mixture of HfO₂, HfSiO₄ and calcium aluminosilicate,Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇, and combinations thereof.
 11. The airfoilas recited in claim 9, wherein the first environmental barrier topcoatis multi-layered and includes an over-layer of a composition that isselected from HfO₂, Y₂SiO₅, YbSiO₅, and combinations thereof and anunder-layer of a composition that is selected from HfSiO₄, Y₂Si₂O₇,Yb₂Si₂O₇, and combinations thereof, and the second environmental barriertopcoat is multi-layered and includes an over-layer of a composition ofGd₂Hf₂O₇ and an under-layer of a composition of a mixture of HfSiO₄ andcalcium aluminosilicate.
 12. The airfoil as recited in claim 9, furthercomprising a fourth environmental barrier topcoat disposed in thetrailing end of the airfoil wall, wherein the fourth environmentalbarrier topcoat is thicker than each of the first environmental barriertopcoat and the second environmental barrier topcoat.
 13. The airfoil asrecited in claim 9, further comprising a fourth environmental barriertopcoat disposed in the trailing end of the airfoil wall, wherein thefourth environmental barrier topcoat has, by volume percent, a higherporosity than each of the first environmental barrier topcoat and thesecond environmental barrier topcoat.
 14. An airfoil comprising: anairfoil wall defining a leading end including an apex, a trailing end,and suction and pressure sides joining the leading end and the trailingend, the airfoil wall being formed of a silicon-containing ceramic; avaporization-resistant environmental barrier topcoat of a firstcomposition disposed on the suction side of the airfoil wall, thevaporization-resistant environmental barrier topcoat initiating at theleading end at a first distance from the apex; and acalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat of a second, different composition disposed on the pressure sideof the airfoil wall, the calcium-magnesium-aluminosilicate-resistantenvironmental barrier topcoat initiating at the leading end at a seconddistance from the apex
 15. The airfoil as recited in claim 14, whereinthe first distance is from 0.5 millimeters to 7.7 millimeters and thesecond distance is from 0.5 millimeters to 7.7 millimeters.
 16. Theairfoil as recited in claim 15, wherein the first distance is from 1.2millimeters to 3 millimeters and the second distance is from 1.2millimeters to 3 millimeters.
 17. The airfoil as recited in claim 16,wherein the vaporization-resistant environmental barrier topcoat is of acomposition that is selected from HfO₂, rare earth monosilicate, HfSiO₄,Y₂Si₂O₇, Yb₂Si₂O₇, alkaline earth alumino-silicates (AEAl₂Si₂O₈), andcombinations thereof, and thecalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat is of a composition that is selected from a mixture of HfSiO₄and calcium aluminosilicate, Ca₂Y₈(SiO₄)₆O₂, Gd₂Hf₂O₇, a mixture ofHfO₂, HfSiO₄ and calcium aluminosilicate, Y₂Si₂O₇, Yb₂Si₂O₇, Gd₂Si₂O₇,and combinations thereof.
 18. The airfoil as recited in claim 17,wherein the vaporization-resistant environmental barrier topcoat ismulti-layered and includes an over-layer of a composition that isselected from HfO₂, Y₂SiO₅, YbSiO₅, and combinations thereof and anunder-layer of a composition that is selected from HfSiO₄, Y₂Si₂O₇,Yb₂Si₂O₇, and combinations thereof, and thecalcium-magnesium-aluminosilicate-resistant environmental barriertopcoat is multi-layered and includes an over-layer of a composition ofGd₂Hf₂O₇ and an under-layer of a composition of HfSiO₄ and calciumaluminosilicate.
 19. The airfoil as recited in claim 14, furthercomprising an additional environmental barrier topcoat disposed on thetrailing end of the airfoil wall, the additional environmental barriercoating initiating at a distance from a mechanical trailing edge of theairfoil that is greater than the thickness of the mechanical trailingedge by a factor of 2 to
 10. 20. A gas turbine engine comprising: acompressor section; a combustor in fluid communication with thecompressor section; and a turbine section in fluid communication withthe combustor, the turbine section having an airfoil that includes anairfoil wall defining a leading end, a trailing end, and suction andpressure sides joining the leading end and the trailing end, the airfoilwall being formed of a silicon-containing ceramic, a first environmentalbarrier topcoat of a first composition disposed on the suction side ofthe airfoil wall, and a second environmental barrier topcoat of asecond, different composition disposed on the pressure side of theairfoil wall.